Recently, there have been extensive advancements in research and standardization to realize multi-Gigabit wireless transmission over 60 GHz carrier such as WirelessHD, WiGig and IEEE802.15.3c. In these standards, beamforming is exclusively considered due to the fact that LoS channels are in general rank deficient and beamforming is efficient to boost signal-to-interference-and-noise-ratio (SINR) for 60 GHz wireless channel with high path loss. Traditional spatial multiplexing techniques operating with rich scattering and multipath for lower carrier frequency is in general not considered in 60 GHz transmission.
The criteria to create high rank MIMO channel in a green field (or LoS) environment has been initially reported and shown that rank properties of LoS wireless link are governed by simple geometrical propagation parameters such as antenna spacing and transmitter-receiver (Tx-Rx) distance. Some recent investigations continued to show that the spatial multiplexing gain can be obtained in LoS channels by employing specifically designed antenna arrays to preserve the orthogonality between antenna signatures. Briefly, high rank or full rank MIMO channel can be created by increasing inter-element antenna spacing and assuming particular geometrical setting (Tx-Rx distance and antenna array orientation). The above exciting works show that the high rank MIMO transmission is theoretically feasible for 60 GHz LoS channel. However, in above works, the knowledge of the exact geometrical placement is required to realize the high rank/full rank MIMO transmission. Thus neither of them has much practical value because the actual placement is very difficult to be predicted and the antenna spacing usually cannot be dynamically adjusted once it is deployed. Moreover, there is no beamforming gain in the proposed approach.
A system architecture supporting a heuristic combination of beam-forming and spatial multiplexing has been disclosed. However, it requires full channel state information at the receiver to derive the transmit and receive beamforming vectors, which is a quite impractical assumption for 60 GHz systems especially for the cases with a large number of transmitter or receiver antennas. Moreover, without rank adaptation capability, this system is quite sensitive to geometrical change, misplacement and human blockage effect. There are also other efforts of creating high rank MIMO in LoS channel, such as the method based on polarization diversity and the repeater-assisted capacity enhancement scheme. However, neither is applicable in practice so far.
Accordingly, there is a need for a practical MIMO transmission with rank adaptation in the 60 GHz wireless radio link.